“No doubt he is right,” said Thorndyke.  “But now, tell me, do you think it is possible for any person to board or leave the train on the off-side unobserved?  Could a man, for instance, enter a compartment on the off-side at one station and drop off as the train was slowing down at the next, without being seen?”

“I doubt it,” replied the station-master.  “Still, I wouldn’t say it is impossible.”

“Thank you.  Oh, and there’s another question.  You have a gang of men at work on the line, I see.  Now, do those men belong to the district?”

“No, sir; they are strangers, every one, and pretty rough diamonds some of ’em are.  But I shouldn’t say there was any real harm in ’em.  If you was suspecting any of ’em of being mixed up in this—­”

“I am not,” interrupted Thorndyke rather shortly.  “I suspect nobody; but I wish to get all the facts of the case at the outset.”

“Naturally, sir,” replied the abashed official; and we pursued our way in silence.

“Do you remember, by the way,” said Thorndyke, as we approached the empty coach, “whether the off-side door of the compartment was closed and locked when the body was discovered?”

“It was closed, sir, but not locked.  Why, sir, did you think—?”

“Nothing, nothing.  The sealed compartment is the one, of course?”

Without waiting for a reply, he commenced his survey of the coach, while I gently restrained our two companions from shadowing him, as they were disposed to do.  The off-side footboard occupied his attention specially, and when he had scrutinized minutely the part opposite the fatal compartment, he walked slowly from end to end with his eyes but a few inches from its surface, as though he was searching for something.

Near what had been the rear end he stopped, and drew from his pocket a piece of paper; then, with a moistened finger-tip he picked up from the footboard some evidently minute object, which he carefully transferred to the paper, folding the latter and placing it in his pocket-book.

He next mounted the footboard, and, having peered in through the window of the sealed compartment, produced from his pocket a small insufflator or powder-blower, with which he blew a stream of impalpable smoke-like powder on to the edges of the middle window, bestowing the closest attention on the irregular dusty patches in which it settled, and even measuring one on the jamb of the window with a pocket-rule.  At length he stepped down, and, having carefully looked over the near-side footboard, announced that he had finished for the present.

As we were returning down the line, we passed a working man, who seemed to be viewing the chairs and sleepers with more than casual interest.

“That, I suppose, is one of the plate-layers?” Thorndyke suggested to the station-master.

“Yes, the foreman of the gang,” was the reply.

“I’ll just step back and have a word with him, if you will walk on slowly.”  And my colleague turned back briskly and overtook the man, with whom he remained in conversation for some minutes.